This invention relates to a differential amplifier circuit.
If a differential voltage is applied on the input of a transconductance amplifier, the amplifier sinks current from an output pin of the circuit, through a current sinking transistor to ground. The output voltage (an amplified version of the differential input) is developed across a load resistor connected to the output pin. When the transconductance amplifier is implemented on a chip, in order not to damage the chip it is advisable to limit the sinking current to a predetermined value.
According to the present invention there is provided a differential amplifier circuit comprising: an amplifying section for generating at an output node an amplified output dependent on a differential input voltage, said amplifying section including first and second current branches, each including a respective transistor, said transistors having their bases connected together and parallel collector paths an output stage comprising a current sinking element having a control node, and a controllable path connected to the output node, and a control element having a control node connected to the collector path of the second current branch and a controllable path connected to the control node of the current sinking element; and a current limiting section comprising a current detecting element connected to detect the current through the current sinking element and arranged to drive a current limiting element when the detected current exceeds a predetermined threshold to inject current at the collector of the transistor in the first current branch.
The current limiting section serves to limit the current sunk through the current sinking element by current feedback into its control node. In the described embodiment, the current sinking element, the control element, the current detecting element and the current limiting element are all implemented as bipolar transistors.
For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings.